The present invention relates generally to the application of charge-transfer materials to nonlinear optics and, more particularly, to femtosecond nondegenerate four-wave mixing in donor-acceptor material blends for generating ultrafast holographic effects. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy to The Regents of the University of California and under Contract No. F49620-95-0395 awarded by the Air Force Office of Scientific Research to The Regents of the University of California. The government has certain rights in the invention.
Holographic gratings are a principal component of dynamical optical systems based on nonlinear optical materials, and are useful in a variety of optical applications, including interconnection networks, optical memories, and optical computing. Inorganic photorefractive crystals have been the most widely studied materials for such applications. For descriptions of applications of holographic nonlinear optical materials, see, e.g., E. S. Maniloff and K. M. Johnson, xe2x80x9cMaximized Photorefractive Holographic Storagexe2x80x9d, J. Appl. Phys. 70, 4702 (1991). Recently, however, organic holographic materials such as photorefractive polymers, photochromic molecules, and semiconducting polymers, have received considerable attention. See, e.g., W. E. Moerner and S. M. Silence, xe2x80x9cPolymeric Photorefractive Materials,xe2x80x9d Chem. Revs. 94, 127 (1994), for a discussion concerning slow photorefractive polymeric materials. One of the major problems constraining the practical use of holographic materials has been the trade-off between speed and diffraction efficiency inherent in many classes of materials. The class of materials with the highest diffraction efficiencies has been photorefractives; however, the response time of these materials has been limited by diffusion (or drift) rates. Third-order ("khgr"(3)) nonlinear optical materials can have essentially instantaneous response times, but have low diffraction efficiencies.
The optical properties of semiconducting polymers are significantly changed with the addition of buckminsterfullerene, C60. After photoexcitation across the xcfx80-xcfx80* gap, an electron transfers from the polymer (as donor) to the C60 (as acceptor). The charge-transfer process is ultrafast, occurring within 300 fs, with a quantum efficiency approaching unity. See, e.g., N. S. Sariciftci et al., Science 258, 1474 (1992) for a discussion of donor-acceptor photoinduced charge transfer. As a result of the efficient photoinduced intermolecular charge transfer, the photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectral features of the composite films can be significantly enhanced in magnitude over those in either of the component materials. The corresponding changes in the complex refractive index, xcex94N=xcex94n(xcfx89)+"igr"xcex94xcexa(xcfx89), imply that charge-transfer mixtures offer promise as nonlinear optical materials, i.e., as holographic materials with absorption gratings in spectral regions where xcex94xcexa(xcfx89) dominates and as holographic materials with index gratings in spectral regions where xcex94n(xcfx89) dominates. Holographic recording using photoinduced charge transfer has a number of characteristics which distinguish it from other materials discussed in the literature: 1) The materials respond on a femtosecond timescale, 2) A larger diffraction efficiency is achieved than any previous report using ultrafast materials, and 3) Control of the holographic relaxation rate is achieved by use of a two-component recording mechanism.
A comparison of the maximum diffraction efficiency or the response time of different materials does not allow an adequate evaluation of their relative merits, since rapid data processing requires having both a large response and a rapid recording rate. As a figure-of-merit for comparing previous research, the temporal diffraction efficiency (TDE) is defined as xcex7/xcfx84, with xcex7 being the diffraction efficiency and xcfx84 being the time constant governing the holographic buildup. The TDE gives a measure of how fast and how strongly a material responds to the recording waves and, therefore, of how rapidly the material can be expected to be used for data processing. When using this figure of merit, it is important to note the intensity at which the measurement is made as well as the saturation diffraction efficiency, since increasing the recording intensity will affect the rate at which a grating is recorded, and a high value of the TDE does not necessarily imply that a material has sufficiently high maximum response for a particular application. As an example, photorefractive materials have large efficiencies (approaching unity), but because they respond on timescales xe2x89xa71 s, they have TDE values xe2x89xa61 sxe2x88x921, for intensities of approximately 1 W/cm2.
A large number of materials that undergo photoisomerization have recently been reported, and suggested as possible elements for dynamic holographic processing. The TDE of these materials is in the range of 10xe2x88x921-10xe2x88x926 sxe2x88x921, with recording intensities typically in the range of 10-50 mW/cm2. For example, V. Pham et al., in Opt. Mat. 4, 467 (1995), report the maximum diffraction efficiency as 5%, with a recording time constant of 3.2 s for an intensity of 19 mW/cm2, corresponding to a TDE of 1.6xc3x9710xe2x88x922. Studies of photoisomerization has shown that by using nondegenerate four-wave mixing (NDFWM) leads to a significant improvement in diffraction efficiencies.
Third-order nonlinearities have been extensively used for degenerate four-wave mixing in organics and for demonstrations of optical processing. Because of the ultrafast response of these materials, TDE values as high as 109 sxe2x88x921 (xcex7≈10xe2x88x924 in 160 fs) have been reported for pulse energies of ≈5 xcexcJ (500 xcexcJ/cm2). As a mechanism for incoherent third-order nonlinearity, "khgr"inc(3), the photoinduced electron transfer of semiconducting polymer/C60 mixtures offers two important advantages: the metastability of the charge transfer enables control of the grating decay dynamics by varying the concentration of acceptors in the mixture, and the photoinduced charge transfer enhances the magnitude of the modulated changes in the complex index of refraction at certain wavelengths.
Accordingly, it is an object of the present invention to photoinduce large changes in the complex index of refraction for mixtures of donor and acceptor charge-transfer species such that ultrafast holographic gratings are generated.
Another object of the invention is to photoinduce large changes in the complex index of refraction of mixtures in blends of semiconducting polymers with C60 such that ultrafast holographic gratings are generated.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method for nondegenerate four-wave mixing in charge-transfer materials of this invention includes the steps of preparing a mixture of charge-donor species capable of absorbing light within a first wavelength region with charge-acceptor species, such that charge transfer occurs between the charge-donor species and the charge-acceptor species in the absence of an externally applied dc electric field when the mixture is excited by light in the first wavelength region; exciting the mixture with two light beams within the first wavelength region, the two light beams overlapping in the blend and further having an angle therebetween, whereby a holographic grating is formed in a region of the mixture surrounding the overlapping of the two light beams; passing light within a second wavelength region through the holographic grating region of the mixture, wherein the second wavelength region is chosen such that light therein is not absorbed by the mixture in the absence of charge transfer; and detecting the diffracted light within the second wavelength region.
Benefits and advantages of the invention include enhancement of the magnitude of modulated changes in the complex index of refraction at certain wavelengths, and control of the decay dynamics through variation of the concentration of acceptors in the mixture.